Ion channels are instrumental in the generation of membrane potential, receptor potential, and action potential. They are the molecular building blocks for what is an essential characteristic of many cells in neuroscience: excitability. These proteins are implicated in the physiology and pathophysiology of all excitable tissues, underlie many disease processes including epilepsies and arrhythmias, and are major targets of essential drugs used in clinical medicine. Potassium channels are the phylogenetic founders of a large superfamily of structurally related ion channels that includes nucleotide gated channels, sodium channels, and calcium channels. Potassium channels are typically assembled from four identical subunits in a four-fold symmetrical fashion. This rather simple structural blueprint and the substantial practical advantage of only one subunit type have made potassium channels a much studied model system. Potassium channels exhibit two important properties: they selectively admit potassium over sodium and they change their structure during function in a process called gating. This proposal explores the gating transition in potassium channels. We will use a combination of cysteine scanning mutagenesis, site-directed spin labeling, site-directed mass tagging, X-ray crystallography, and electrophysiology to study the gating transition in three prokaryotic potassium channels: KcsA, MthK, and KvAP. The long term goal of this proposal is to understand functional properties of ion channels at a structural level.